Tag Archives: diffusion

Malteser and sardine salad


I must say that I’m not overly impressed with WordPress. For one thing, you’ve probably noticed an annoying box at the top of my home page. No idea how it got there, no idea how to get rid of it. Grrrr. And then, when I tried to form a Word Cloud to help people find certain topics or resources, it took me hours to achieve absolutely nothing. Double grrrr.

Anyway, it’s achieving its main function of hosting my burbles and linking them to specific resources. And it’s free, so I really shouldn’t grumble.

I’m going to stick with Year 9s this week. Bill’s flipped diffusion worked brilliantly, as did the follow up, and they can now describe and explain diffusion based largely on what they have observed and deduced. My one modification for next year is to include a classic “cheat” that I’m sure we’re all guilty of – the warm/cold diffusion rate was different, but not as much as you might expect. So next year I’ll be providing 0.1M cold acid to compare with the 1M warm acid (though, of course, they will be told that the concentrations are equal). I’d love to hear other examples of this kind of educational economy with the truth – it would make a good post!

Next up, we’re going to review why diffusion is important. I like to use this Powerpoint sardine simpler version – another Bill creation – for them to observe and interpret. What’s going on here? It’s very neat. Notice, incidentally, how creative you can be with this much maligned software, once you’ve learned some of the basic animation features. Notice, too, the complete lack of notes! There’s a few questions to go with this, linking back to the microscope work they’ve done on Protoctists a few weeks back meet Mr Paramecium .

I then ask for a volunteer. They might be hesitant as they’re not sure what to expect, but I reassure them. I tell them they’re an amoeba. I remind them that they need a constant supply of oxygen. I produce a box of maltesers. Each malteser represents oxygen. We imagine that they need one oxygen every 3 seconds to survive. I then shovel maltesers into them at the rate of around 1 every 3 seconds. All good fun. The idea is that a single celled organism can rely on simple diffusion from the environment for its oxygen demands.

I then ask for 10 volunteers. No hesitancy now! I line them up in a row, with the original amoeba at the far end. Now they’re cells in a multi-cellular organism. Same rules apply. They each need a malteser every 3 seconds. Can they still rely on diffusion from the outside world? It’s great – the maltesers never get past the 2nd or 3rd “cell” – so they immediately see the problem – it’s too slow, and the oxygen all gets used up anyway.

Back to the powerpoint! Look at the sardine and the idea of an oxygen transport system. And then they all dissect a sardine (or mackerel, or herring, or whatever comes off the slab!). The excitement over this activity is astonishing! I like to set little challenges, so as well as getting them to cut out the gills and dip them in water/observe under the binocular microscope, I encourage them to try and find the swim bladder without bursting it, to find the heart, to not reduce the fish itself to a pile of mangled sushi… Labelling and annotating this diagram Why a Sardine cannot rely on Diffusion alone for its oxygen supply can always come later…

Right, I’ve got a train to catch. More next week!

Year 9 diffusion

Good morning!

My Year 9s have finished their animal/plant cell introduction to iGCSE with an exercise where they pretend to be Martian scientists who have taken two samples of life from Earth – a geranium and a student – and are trying to figure out whether they’re just slightly different versions of the same thing, or whether they are fundamentally different. It’s a fun way of getting them to work out the key classification points, without actually telling them.

They’re now into diffusion as a logical extension of this – what do cells need? And how do they get these things? Always keen to try something new, I started this year with Bill’s exercise (attached). Like all great ideas, it’s deceptively simple. But look at it in a bit more detail, you start to see not only why is it fun, engaging and challenging, but also why it works so well as a learning activity. Note the lack of preamble, no “today we’re going to study diffusion…” introduction, just straight into a “do this and figure it out” approach.

It’s fun – they love the little cubes (though they were disappointed to learn about their lack of edibility) and are genuinely intrigued by the colour change. They can explain this at one level – the challenge is to push them beyond the “acid is moving into the cube” description, to something more rigorous about particles. But they quickly see the effect of concentration and by the end of the activity they’ve basically worked out the definition of diffusion for themselves, and seen/measured that it happens more quickly if the difference in concentration is greater.

I’m following this up with an adaptation of my olde introduction to diffusion (attached). As well as making lots of different sized cells, they’re also going to compare diffusion rates at two different temperatures. Again, they work it all out for themselves. The questions are for homework.

Next week I’ll tell you how I move this on to multi-cellular organisms and maltesers…

Have a great week/weekend!


1LT1 – Flipped diffusion in jelly blocks 1

Acid bath and agar blobs with temperature NOV 2014